Fuel tank caps, also called filler cap assemblies, may be used for sealing the opening of a fuel tank of a vehicle, such as a commercial trucking vehicle. Fuel caps typically are placed on the fuel tank opening and are locked in place to create an airtight seal over the opening. It may be desirable that the airtight seal be maintained during operation of the vehicle so that fuel or fuel vapor is not lost to the atmosphere and so that spillage of the fuel does not occur. Accordingly, it may be desirable that the seal and the locking mechanism be sturdy enough to withstand the harsh environmental conditions of commercial vehicle usage such as extreme weather conditions and continued vibration of the fuel tank and cap during operation of the vehicle. Moreover, it may be desirable that fuel caps maintain an airtight seal over the fuel tank opening during a sudden impact to the fuel tank, such as during a collision, so that fuel is not discharged, which may result in a fire or an explosion.